This invention relates generally to thin film batteries, and more particularly to a method of producing thin film, rechargeable lithium ion batteries.
Conventional, canister type batteries today include toxic materials such as cadmium, mercury, lead and acid electrolytes. These chemicals are presently facing governmental regulations or bans as manufacturing materials, thus limiting their use as battery components. Another limitation associated with them is that the amount of energy stored and delivered by these batteries is directly related to their size and weight. Large batteries, such as those found in automobiles, produce large amounts of current but have very low energy densities (Watts hours per liter) and specific energies (Watt hours per gram). As such, they require lengthy recharge times which render them impractical for many uses.
To address the need for higher energy densities and specific energies, the battery industry has been moving towards lithium based batteries. The major focus of the battery industry has been on liquid and polymer electrolyte systems. However, these systems have inherent safety problems because of the volatile nature of the electrolyte solvents. These types of batteries have a relatively high ratio of inert material components, such as the current collector, separator, and substrate, relative to the active energy storage materials used for the anode and cathode. In addition, their relatively high internal impedance results in low rate capability (watts/kilogram) which renders them impractical for many applications.
Thin film lithium batteries have been produced which have a stacked configuration of films commencing with an inert ceramic substrate upon which a cathode current collector and cathode are mounted. A solid state electrolyte is deposited upon the cathode, an anode in turn deposited upon the electrolyte, and an anode current collector mounted upon the anode. Typically, a protective coating is applied over the entire cell. Lithium batteries of this type are describe in detail in U.S. Pat. Nos. 5,569,520 and 5,597,660, the disclosures of which are specifically incorporated herein. These lithiated cathode material of these batteries have a (003) alignment of the lithium cells, as shown in FIG. 1, which creates a high internal cell resistance resulting in large capacity losses.
Recently, it has been discovered that the annealing of lithiated cathode materials on a substrate under proper conditions results in batteries having significantly enhanced performances, for the annealing causes the lithiated material to crystallize. This crystallized material has a hexagonal layered structure in which alternating planes containing Li and Co ions are separated by close packed oxygen layers. It has been discovered that LiCoO2 films deposited onto an alumina substrate by magnetron sputtering and crystallized by annealing at 700xc2x0 C. exhibit a high degree of preferred orientation or texturing with the layers of the oxygen, cobalt and lithium are oriented generally normal to the substrate, i.e. the (101) plane as shown in FIG. 2. This orientation is preferred as it provides for high lithium ion diffusion through the cathode since the lithium planes are aligned parallel to the direction of current flow. It is believed that the preferred orientation is formed because the extreme heating during annealing creates a large volume strain energy oriented generally parallel to the underlying rigid substrate surface. As the crystals form they naturally grow in the direction of the least energy strain, as such the annealing process and its resulting volume strain energy promotes crystal growth in a direction generally normal to the underlying substrate surface, which also is the preferred orientation for ion diffusion through the crystal.
However, the limitations of these batteries have been the thickness and weight of their substrates upon which the layers of active material are laid upon. Because of the size of the substrate, these batteries have not been competitive with other formulations in terms of energy density and specific energy. High energy density cells have not been successfully constructed. The supporting substrates have been made of relatively thick sheets of alumina, sapphire, silica glass and various other types of ceramic material. The current collector and substrate of these batteries typically constitute nearly 70% of the total weight and an even larger percentage of the volume, thus only a small amount of the of the battery weight and volume is attributed to the active materials. This ratio of active material to the overall weight and volume of the battery limits their use. The performance of these batteries are also limited by the amount of surface area interfacing between adjacent components, such as between the cathode current collector and cathode, between the cathode and electrolyte, between the electrolyte and anode, and between the anode and anode current collector. This limited area of interface degrades the current flow through the battery.
To increase the interface area some batteries have been developed wherein a current collector has two opposite sides coupled to the reactive material. As shown in U.S. Pat. No. 4,092,464, these batteries are typically manufactured by simply folding over each layer of component over each successive interior layer. As each component is assembled by folding over the prior component the connection between each successive layer may not be continuous, i.e. there may be gaps or incompleteness in the connection between layers. These gaps again limit the performance of the battery.
It thus is seen that a need remains for a high performance rechargeable, thin film lithium battery which is smaller and lighter than those of the prior art and which may be manufactured to optimize the interface between successive components. Accordingly, it is to the provision of such that the present invention is primarily directed.
In a preferred form of the invention, a method of manufacturing a thin film battery comprises the steps of providing a substantially planar cathode current collector having two oppositely disposed generally planar surfaces, depositing a first cathode layer upon one cathode current collector surface, depositing a second cathode layer upon the other cathode current collector surface, depositing a first electrolyte layer upon the first cathode layer, depositing a second electrolyte layer upon the second cathode layer, depositing a first anode layer upon the first electrolyte layer, and depositing a second anode layer upon the second electrolyte layer.